The apparatus includes: At least two ultrasonic sensors, which are secured in a defined measuring positional relationship, i.e. in defined measuring positions, externally on the pipeline and which alternately emit and receive ultrasonic measuring signals; and a control/evaluation unit, which determines the volume- and/or mass-flow of the medium in the pipeline on the basis of the travel time difference of the ultrasonic measuring signals, in the stream direction, and opposed to the stream direction.
Clamp-on ultrasonic flow measuring devices of the above-described kind, which determine volume flow by means of the so-called travel time difference method, are employed for many uses in process and automation technology. They make it possible to determine volume flow contactlessly in a containment, e.g. in a pipeline. To this end, the ultrasonic measuring signals are radiated at a predetermined angle into the pipeline, in which the medium is located. The measuring positional relationship of the ultrasonic sensors on the pipeline depends on the inner diameter of the pipeline, on the wall thickness thereof, and on the velocity of sound in the material of the pipeline. In order to provide a reliable measured variable for the flow, these parameters must be known.
Known are clamp-on flow measuring devices in which the ultrasonic transducers are pressed externally onto the pipeline by means of a clamping lock. Clamp-on flow measuring devices are described, for example, in European patent EP 0 686 255 B1, U.S. pat. No. 4,484,478 or U.S. pat. No. 4,598,593. Additionally, it is known to apply the clamp-on measuring devices to the pipeline using chains, hook and loop bands (e.g. Velcro hook and loop bands), or screws. It is clear that the known methods for positioning ultrasonic sensors are rather time consuming.